PMD-pump mounted driver or FSD-fuel solenoid driver,this is the black/grey box on the drivers side of the injection pump in stock form.
It is an electronic switchgear that amplifies ECM pulses to a usable current to run the IP fuel solenoid via transistors and other electronics.
They often fail and a common replacement option is to remotely locate it to a location outside the engine bay on an aluminum heatsink with an extension harness.
PMD's fail due to repeated heat cycling breaking internal solder joints that are not user servicable.
The best solution to date is to get them away from external heat sources and to shed the heat they generate when operating to the outside air so that they stay at or near ambient temps at all times.
Often a replacement PMD resistor is required at relocation time due to the difficulty of retrieving the original from the the IP mounted PMD.